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\section{Introduction}   Importance of Carbon   There are 2,300 Pg of carbon (C) stored in soils worldwide, excluding plant biomass, which accounts for \sim80\% of the global terrestrial C pool (\cite{Amundson_2001}, IPCC 2000, IPCC 2007, \cite{elsen_Ayres_Wall_Bardgett_2011}, http://rstb.royalsocietypublishing.org/content/363/1492/815.full - \cite{Lal_2008}, \cite{BATJES_1996}). It is estimated that 80-90\% of the C cycling in soil is mediated by microorganisms (Coleman & Crossley 1996, Nannipieri & Badalucco 2003), however, 2003). However,  understanding microbial processing of nutrients in soils presents a special challenge due to the hetergeneous nature of soil ecosystems and our limitations in methodologies. Soils consist of an overwhelming biological, chemical, and physical complexity which affect microbial community composition, diversity, and structure (refs). That and the confounding factors such as physical protection/aggregation, moisture content of the soil, pH, temperature, frequency and type of land disturbance, soil history, mineralogy, litter quality, and N quality and availability that dictate the fate of C (Gessner et al 2010, Sollins et al 1996, \cite{Torn_Vitousek_Trumbore_2005}, \cite{TRUMBORE_2006}) all contribute the challenges of studying soil carbon dynamics. These factors have all been shown to affect the ability of the soil microbial community to access and metabolize C substrates (Schlesinger 1977). Furthermore, rates of metabolism are often measured without knowing the identity of the microbial species specifically involved in the cycling of the measured process (\cite{ndi_Pietramellara_Renella_2003}). Therefore, the first step in teasing out this central problem is to identify microbial groups responsible for the measured process and understand the relations between genetic diversity, community structure, and function (O’Donnell et al 2001). The importance of community diversity in maintaining ecosystem functioning remains uncertain (Allison & Martiny 2008, \cite{ndi_Pietramellara_Renella_2003}) The aim of this proposal is to track the path of C added to soil as a complex C mixture that chemically simulates switchgrass biomass, to provide insight into these complex systems. A previous study has shown that 13C labeled plant residues enable tracking of C through microbial pathways (Evershed et al 2006). Utilizing this technique with single 13C labeled substrates added as a complex C mixture will allow us to test how different C containing components move through soil trophic cascades. Powerful techniques such as nucleic acid stable isotope probing (SIP) coupled with 454 pyrosequencing can then be used to separate out and identify these 13C labeled portions of the microbial community to reveal the community members that are responsible for the transformation of C. Coupling “plant simulant” additions to microcosm incubations with nucleic acid stable isotope probing (SIP) may provide a means of looking into microbial community functions while minimizing other factors affecting the fate of C. This will allow a researcher to sift out the specific organisms or functional guilds that are responsible for the degradation and cycling of that specific C substrate. Furthermore, the processing of the C substrate can be traced over time whether it is assimilated into microbial biomass, respired as CO2, or incorporated into recalcitrant soil organic matter.